1. Field of the Invention
The present invention relates to a high-temperature oxide superconductor which has a superconducting transition temperature (referred to as T.sub.c) higher than the boiling point of liquid nitrogen and is less liable to the variation of T.sub.c due to the liberation of oxygen that takes place during fabrication at a high temperature.
2. Description of the Prior Art
Among the known typical oxide superconductors having a T.sub.c higher than the boiling point of liquid nitrogen is RBa.sub.2 Cu.sub.3 O.sub.7 of three-layer perovskite structure which has a T.sub.c of 90 K. (R denotes Y or at least one element selected from the lanthanoid rare earth elements.) [Appl. Phys. Lett. Vol. 51 (1987) P57]
Unfortunately, the above-mentioned oxide superconductor has the property that oxygen atoms as the constituent members are liable to get out of it when it is affected by heat during fabrication. For this reason, the oxygen content in the superconductor varies depending on the heating conditions for fabrication. The variation of oxygen content brings about the orthorhombic-tetragonal phase transition, which in turn causes the T.sub.c to vary over a broad range from 0 K to 90 K. [Phys. Rev. B36 (1987) P5719]
For example, RBa.sub.2 Cu.sub.3 O.sub.7 powder becomes deteriorated in superconductivity due to the partial loss of oxygen atoms which occurs during the sintering heat treatment in the production of a superconducting wire by the silver-sheath wire drawing method. This method involves the steps of filling the oxide semiconductor powder in a silver pipe, working the silver pipe into a wire by cold drawing, and sintering the powder at 800.degree.-900.degree. C.
In contrast with the above-mentioned oxide superconductor, there is known another oxide superconductor of RBa.sub.2 Cu.sub.4 O.sub.8 type, which has the double-layered Cu--O chains instead of the single-layered Cu--O chains in the RBa.sub.2 Cu.sub.3 O.sub.7 -type crystal structure. It has a stable chemical structure, retaining oxygen atoms until it is heated to about 850.degree. C. Moreover, it has the T.sub.c in the neighborhood of 80 K, which is higher than the boiling point of liquid nitrogen. Thus it is regarded as an important substance from the standpoint of practical use.
The oxide superconductor of RBa.sub.2 Cu.sub.4 O.sub.8 type can be produced by one of the following two methods which have been proposed so far.
(1) A method involving the heat treatment of a calcined powder in a high-pressure atmosphere of pure oxygen. (E.g., at 930.degree. C. for 8 hours under an oxygen pressure of 100 atm) [High-pressure oxygen method; T.sub.c =81 K, Nature 336 (1988) P660-662, or Phys. Rev. B39 (1988) p7347-7350]
(2) A method involving the heat treatment of a mixture of a calcined powder with a catalyst (such as sodium carbonate) in an oxygen stream for a long time. (E.g., at 800.degree. C. for 72 hours) [Normal-pressure method; T.sub.c =77 K, Nature 338 (1989) P328-330]
Unfortunately, the superconductor compound of R-Ba-Ca--O type presents a problem associated with its production process because it contains a large portion of Ba and its synthesis needs a large amount of Ba compounds such as BaO, BaCO.sub.3, and Ba(NO.sub.3).sub.2, which are deleterious substances. (For example, the Ba content in YBa.sub.2 Cu.sub.4 O.sub.8 is 13.33 mol %.) Another disadvantage of this oxide superconductor is that it cannot be made with cheap BaCO.sub.3 by the above-mentioned methods because BaCO.sub.3 decomposes at 1000.degree. C. or above under normal pressure, whereas the reaction in the above-mentioned methods takes place at a comparatively low temperature. Therefore, it has to be made with an expensive compound, such as Ba(NO.sub.3).sub.2, in place of BaCO.sub.3.
In addition, the present inventors' investigation revealed that an oxide of YBa.sub.2 Cu.sub.4 O.sub.8 type is poor in sinterability and is liable to give a sintered body having a rather low density. The sintered body of low density has a low critical current density as a matter of course.